Film products fabricated from linear low density polyethylene (LLDPE) or high density polyethylene (HDPE) are widely used for packaging applications such as merchandise bags, shrink films, grocery sacks, and industrial liners. For these applications, films with excellent toughness properties (that is, high tensile, impact or tear strengths) are desired to facilitate down gauging, prevent premature punctures during handling, distribution and use.
Previous attempts were made to optimize film tensile strength and yield strength by blending various heterogeneous polymers together on theoretical basis. While such blends exhibited a synergistic response to increase the film yield strength, the film impact strength followed the rule of mixing, often resulting in a “destructive synergism” (that is, the film impact strength was actually lower than film made from one of the two components used to make the blend).
For example, it is known that while improved modulus linear polyethylene polymer blends can be produced by blending high density polyethylene with a very low density polyethylene (VLDPE), the impact strength of these polymer blends typically follow the rule of mixing (or the “blend rule”). That is, the final composition exhibits properties that are comparable to and predictable from weight average calculations based on the component polymers. For example, see FIG. 5 herein.
It is also known from U.S. Pat. No. 5,677,383, the disclosure of which is incorporated herein by reference, that dramatic synergism results when polymer compositions are made from a homogeneously branched ethylene interpolymer having a high slope of strain hardening coefficient melt-blended with a heterogeneously branched ethylene polymer. Although U.S. Pat. No. 5,677,383 broadly discloses the homogeneously branched ethylene interpolymer and the heterogeneously branched ethylene polymer can comprise various comonomers, all of the presented examples consist of higher alpha olefin interpolymer combinations. Moreover, U.S. Pat. No. 5,677,383 focuses on the slope of strain hardening coefficient of the component polymers and is not directed to the slope of strain hardening coefficient of the final blend combinations.
It is also known that, at the equivalent densities, interpolymers comprised of higher alpha olefins provide improved toughness properties as compared to lower alpha olefin interpolymers. For example, at a density of about 0.935 g/cc, an ethylene/1-octene copolymer will exhibit an intrinsic tear that is about two times higher than that of an ethylene/1-butene copolymer having about the same density.
It is well established that higher alpha olefin interpolymers provide superior toughness. But with periodic scarcities in the availability of higher alpha olefin comonomers, resin producers and fabricators desire broader polymer options for providing polymer compositions characterized by excellent toughness properties. The need is especially great in regard to 1-octene, which is a higher alpha olefin that frequently tends to be in short supply while the lower alpha olefin 1-butene generally tends to be abundantly available. For polymer blends comprising at least two higher alpha olefin interpolymers, there is a particular need to substitute at least one of the higher alpha olefin interpolymers with a lower alpha olefin interpolymer and still retain the excellent toughness properties characteristic of higher alpha olefin polymer blends. For polymer blends comprising at least two higher alpha olefin interpolymers where one is a homogeneously branched ethylene interpolymer and the other is a heterogeneously branched ethylene polymer, there is an especially particular need to substitute at least one of the higher alpha olefin interpolymers with a lower alpha olefin interpolymer and still retain the excellent toughness properties characteristic of these polymer blends.
Surprisingly, we have now discovered that within a certain narrow density range, tailored combinations of at least one interpolymer comprised of a lower carbon comonomer blended with at least one interpolymer comprised of a higher carbon comonomer can provide toughness properties comparable to polymer blends comprised of two interpolymers, each having the same higher carbon comonomer or, alternatively, comparable to single interpolymer compositions where the interpolymer comprises a higher carbon comonomer.